Conventionally, in forming images using toner particles, an electrophotography has been generally used, i.e., an application of the Carlson process. The principle of the electrophotography is described in detail in reference to FIG. 6 through an example of a normal developing system adopted in photocopying machines. In the photocopying machine which employs the Carlson process, a charger 32, an exposure unit 33, a developer unit 34, a transfer unit 35, a fuser 36, a cleaner 37, and an eraser 38 are provided in this order along the circumference of a photoreceptor drum 31 having a photosensitive layer formed on the surface thereof as shown in FIG. 6.
With this arrangement, first, the surface of the photoreceptor drum 31 is uniformly charged by the charger 32 in a dark place. Next, an original image is illuminated on the surface of the photoreceptor drum 31 by the exposure unit 33 so as to remove charges from the illuminated portion, thereby forming an electrostatic latent image on the surface of the photoreceptor drum 31. Thereafter, a toner 39 is made to adhere the electrostatic latent image, the toner 39 being charged by applying thereon a charge having a polarity opposite to the charge on the photoreceptor drum 31 in the developer unit 34, thereby forming a visible image of the toner 39. Further, a copying material 40 is superposed on the visible image. Then, a corona-discharging is carried out by the transfer unit 35 from the back surface of the copying material 40 so as to apply a charge having a polarity opposite to the toner 39. As a result, the toner image is transferred to the copying material 40. Then, using heat and pressure of the fuser 36, the transferred toner image is made permanent on the copying material 40. On the other hand, a residual toner 39a remaining on the photoreceptor drum 31 after the transfer is removed by a cleaner 37. After the discharging operation is carried out from the electrostatic latent image on the photoreceptor drum 31 by projecting thereon a light beam from the eraser 38, the process starting with the charging operation by the charger 32 is repeated, thereby successively forming images.
In the discussed electrophotography, i.e., the application of the Carlson process, normally a corona discharger is adopted for charging the photoreceptor drum 31 or transferring the toner 39 to the copying material 40. However, when the corona discharger is adopted, a high voltage of several kV is required. Moreover, it is likely to be affected by a change in the ambient condition, for example, a change in the charge amount on the surface of the photoreceptor drum 31 due to a temperature change. Furthermore, ozone produced in the process of corona charging results in the problem in terms of an environmental health.
In order to counteract the above-mentioned problem, an image forming method not requiring the corona charging is disclosed in Japanese Laid-Open Publication 4900/1990 (Tokukouhei 2-4900). When adopting the method, as shown in FIG. 7, a photoreceptor 50 is desirably arranged such that a transparent electrically conductive layer 52 made of In.sub.2 O.sub.2, etc., a photoconductive layer 53 made of Se etc., and a dielectric layer 54 made of polyethlene terephtalate film, are laminated in this order on a transparent base 51 made of glass or the like. When a magnet 56 as a toner holder having an electrically conductive and magnetic toner 55 adhering thereto is brought close to the surface of the photoconductor 50, in the mean time, the surface of the photoconductor 50 is exposed from the side of a transparent base 51 while applying voltage between the magnet 56 and the transparent electrically conductive layer 52, a resistance of the photoconductive layer 53 at the illuminated portion drops, whereby a charge is injected under the dielectric layer 54. Then, a strong electric field is applied between the magnet 56 and the photoconductor 50, thereby being injected a charge having polarity opposite to that of the toner 55 corresponding to the exposure area. As a result, the charged toner 55 and the charge injected through the transparent electrically conductive layer 52 become being attracted from one another having the dielectric layer 54 in between by making pairs with charges having opposite polarities. In this way, even when the magnet 56 is moved away from the photoconductor 50, the toner 55 at the exposed portion remains on the surface of the photoconductor 50.
As described, the discussed method enables a toner image to be formed on the surface of the photoconductor 50 without using the corona charging. After the toner image is formed on the surface of the photoconductor 50, the toner image is transferred from the surface of the photoconductor 50 to the surface of the copying material as in the case of the Carlson process. Thereafter, the toner is transported to the fuser which heats up the toner to be melted, thereby the toner image is permanently affixed to the copying material.
However, the photoconductor 50 arranged such that the surface of the transparent base 51 whereon the transparent electrically conductive layer 52, the photoconductive layer 53, and the insulating layer 54 are laminated in this order, may be damaged by repeating the image forming process, especially by the repetitive sliding contact of the toner 55 with the insulating layer 54 provided on the periphery surface, or by the blade-shaped cleaner for removing the residual toner on the surface of the insulating layer 54 after the transfer. Especially, when coating the photosensitive layer 53 with an insulating resin material so as to make the dielectric layer 54, an adhesive characteristic to the surface of the photosensitive layer 53, or solubility with respect to solvent needs to be considered in selecting the material. For this reason, the material to be used in the insulating layer 54 is restricted such as polyethlene terephtalate, and the material excellent in its hard-wearing properties, may not be selected. This presents the problem of a high deterioration rate of the dielectric layer 54 which shortens the life of the photoconductor 50.